The present invention relates to spread spectrum communications, and in particular, to a spread spectrum receiver incorporating complementary code keying (CCK). IEEE 802.11(b) provides a standard for wireless local area network communications using complementary code keying. Receivers capable of demodulating CCK modulated signals require extensive circuitry to correlate received data with the numerous possible code words. Traditionally, each received word is correlated with all possible code words for proper correlation. For a 64-word code set, at least 63 full-scale comparitors are necessary for correlation. This vast number of comparitors results in a major hardware cost. There is a need to efficiently correlate received data with the code words in a CCK code set without requiring the extensive amount of components, semiconductor area, and power associated with correlation architectures requiring full-scale comparitors for each code word.
The present invention groups a set of N, nearly orthogonal, CCK code words into M subgroups, with each subgroup consisting of N/M orthogonal code words. For the 64-ary CCK communication standard for wireless local area networks (WLAN), N is 64 and M is preferably 8. Based on the orthogonal subgrouping, most significant bit (MSB) comparators, instead of full-scale comparators, are used to compare cross-correlations for each subgroup. In the subgroup containing the desired maximum correlation value, all other cross-correlation values, except the maximum, are zero in an ideal case, or very close to zero in a noisy environment due to the selected orthogonality. The maximum correlation value within the subgroup can be distinguished by looking at only the most significant bit or bits among all the cross-correlations. The maximum correlation values from all the subgroups, which will include the global maximum from one of the subgroups, are monitored to select the maximum correlation value and the corresponding code word. In the subgroups that do not contain the global, maximum correlation value, it does not matter which cross-correlation value is picked. The invention yields substantial savings on hardware costs by reducing the amount of components and silicon required to implement correlation circuitry, and also reduces power consumption.